The present invention relates to encryption communication apparatuses for improved security management of communication data and, more particularly, to an encryption communication apparatus for performing encryption communication by dividing information into plural data in page units of a given sheet size of a document original and encrypting the information on a page-by-page basis.
Conventionally, so-called encryption communications are employed to enhance data security in communications among a plurality of terminals including telephones, radiophones, facsimile machines and personal computers.
In a facsimile machine, for example, original image data of a document original to be transmitted is first encrypted by using a secret encryption key, and then the encrypted data is sent out to a communication line. In a recipient facsimile machine, the received encrypted data is decrypted by using the same secret key as employed in the encryption process in the transmitter facsimile machine (or the original image data is reproduced).
In general, the transmitter facsimile machine starts reading the document original for transmission from the upper side thereof. For example, the reading of the document original for transmission is started from a left upper portion A of a document original G as shown in FIG. 8. In accordance with the standard document specification defined by the International Consultation Committee for Telegraph and Telephone (CCITT), the portion A is a blank portion and, therefore, the first portion of encrypted data to be transmitted corresponds to the first portion of the decrypted data which further corresponds to the blank portion of the original document.
Accordingly, a third party intercepting the first portion of the transmitted encrypted data can know both the decrypted data corresponding to the blank portion and the encrypted data corresponding to the decrypted data and easily analyze the correlation between the decrypted data 5 and the encrypted data (i.e., encryption algorithm). This allows the encrypted data to be easily decrypted, thereby endangering the communication security.
To cope with this problem, it may be proposed that one or more pixels Db, preliminarily designated in the left upper portion A of the original document G to be first read, are subjected to a black-and-white reversion process before the data encryption and data transmission. Since the first portion of the transmitted encrypted data does not perfectly correspond to the decrypted data corresponding to the blank portion, the encrypted data cannot be easily decrypted.
In accordance with this art, however, part of the original image data corresponding to specific pixels of the document original is subjected to the black-and-white reversion process, and if a document original of plural pages is to be transmitted, the first portions of the encrypted data corresponding to first transmitted blank portions of the second and later pages of the document original are all the same. Therefore, the encryption algorithm used therein can easily be analyzed for decryption of the data.
Further, in PC communications which allow a personal computer to transmit PC data to a recipient facsimile machine via a recently commercialized fax modem, the aforesaid problem may occur when the data encrypted on a page-by-page basis are transmitted.